IZUMO sperm-egg fusion protein 1of 350 aas and 2 TMSs (N- and C-termini). Essential sperm cell-surface protein required for fertilization by acting as a ligand for the IZUMO1R/JUNO receptor on the egg. The IZUMO1:IZUMO1R/JUNO interaction is a necessary adhesion event between sperm and egg that is required for fertilization but is not sufficient for cell fusion (Inoue et al. 2013). The ligand-receptor interaction probably does not act as a membrane 'fusogen' (Klinovska et al. 2014).